Many thermal imaging (i.e., infrared) cameras today produce an image (IR image) of a scene using only energy in the far-infrared portion of the electromagnetic spectrum, typically in the 8-14 micron range. Images obtained using these cameras assign colors or gray-levels to the pixels composing the scene based on the intensity of the IR radiation reaching the camera's sensor elements. Because the resulting IR image is based on the target's temperature, and because the colors or levels displayed by the camera do not typically correspond to the visible-light colors of the scene, it can be difficult, especially for novice users of such a device, to accurately relate features of interest (e.g. hot spots) in the IR scene with their corresponding locations in the visible-light scene viewed by the operator. In applications where the infrared scene contrast is low, infrared-only images may be especially difficult to interpret.
An infrared scene is a result of thermal emission and, not all, but most infrared scenes are by their very nature less sharp compared to visible images which are a result of reflected visible-light. For example, considering an electric control panel of an industrial machine which has many electrical components and interconnections, the visible image will be sharp and clear due to the different colors and well defined shapes. The infrared image may appear less sharp due to the transfer of heat from the hot part or parts to adjacent parts.
When panning an area with a thermal imaging camera looking for hot or cold spots, one can watch the camera display for a visible color change. However, sometimes the hot or cold spot may be small and the color change may go unnoticed. To aid in the identification of hot or cold spots, thermal imaging cameras often indicate the hot spot or cold spot location via a visible cursor or other graphical indicator on the display. The temperature of such hot spots, calculated using well-known radiometric techniques (e.g., establishing or measuring a reference temperature), is often displayed nearby the cursor. Even with the color change and the hot spot indications, it can be difficult to accurately relate the hot spot (or other features of interest) in the camera display's IR imagery with their corresponding locations in the visible-light scene viewed by the operator.
To address this problem of better identifying temperature spots of interest, some cameras allow the operator to capture a visible-light image (often called a “control image”) of the scene using a separate visible-light camera built into the thermal imaging camera. Some of these thermal imaging cameras allow a user to view the visible-light image side-by-side with the infrared image. However, it is often left to the operator to visually correlate image features of interest in the infrared image with corresponding image features in the visible-light image. To make this comparison easier, some thermal imaging cameras now provide simultaneous viewing of the infrared image and the visible-light image overlapping each other and blended together. For example, Fluke Corporation's FlexCam® series of cameras incorporates a feature called IR-Fusion®, which allows a user to blend the infrared and visible-light images together at any ratio from 100% visible to 100% infrared.
Even when the infrared image and the corresponding visible-light image are overlayed and blended, identifying objects within the images can be difficult due to problems in image alignment. For example, in some cases a parallax error may exist due to the physical placement of the infrared and the visible-light sensor within the same (or different) thermal imaging camera. Accordingly, there exists a desire for improved identification systems, including improved alignment or registration of infrared images and visible-light images in thermal imaging cameras.